This invention relates to a cylindrical magnet apparatus for producing a magnetic field in a predetermined direction within the cylindrical bore of the apparatus. Essentially the apparatus is a coaxial assembly of a plurality of dipole ring magnets, and each ring magnet is constructed of a plurality of annularly arranged segments each of which is an anisotropic magnet block. The apparatus is suitable for use in nuclear magnetic resonance (NMR) imaging, and particularly in computerized tomography (CT) apparatus utilizing NMR.
In NMR-CT apparatus the resolution of obtained images depends significantly on the uniformity of the magnetic field in the apparatus. To produce a uniform magnetic field in a sufficiently wide region of a cylindrical space large enough to accomodate a major portion of the human body, it is known to use a plurality of coaxially arranged dipole ring magnets. Since it is impracticable to produce unitary ring magnets large enough for this purpose, it is necessary to divide each of the required dipole ring magnets into a plurality of segments. That is, each dipole ring magnet is formed by annularly assembling a plurality of anisotropic magnet blocks which are respectively magnetized in suitable directions.
As to the magnet material for the aforementioned dipole ring magnets either a conventional ferrite magnet or a rare earth alloy magnet is used. When a ferrite magnet is used the ring magnets need to be made very large in outer diameters so that the gross weight of the assembly of the ring magnets become very heavy. In the case of using a rare earth alloy metal it is possible to greatly reduce the total weight of the ring magnets, but nevertheless there arises a great increase in the material cost because of very high price of the rare earth alloy magnet per unit weight.